1. Field of the Invention
The present invention relates to a fingerprint sensing device comprising an array of sense elements which each comprise a sense electrode. A first switching device is connected to the sense electrode. A first address conductor is operable by a control signal supplied to the first address conductor, from a drive circuit, to apply charge to the sense electrode. The sense electrode of the sense element is spaced from a sensing surface over which is placed a finger whose print is to be sensed. The sense electrode provides in combination with an individual portion of an overlying finger a capacitance. The sense means is connected to each sense element via a second address conductor for providing an output indicative of the capacitances of the sense elements. The invention relates also to a fingerprint recognition system incorporating such a device.
2. Description of the Related Art
A fingerprint sensing device of the above kind is described in U.S. Pat. No. 5,325,442. In this device, the sense elements are arranged in a row and column array and the switching devices of the sense elements comprise thin film transistors, TFTs, connected to a drive circuit via sets of row and column address conductors. The gates of the TFTs of the sense elements in one row are connected to a respective, common, row conductor while the sources of the TFTs of all sense elements in one column are connected to a respective, common, column address conductor. The drain electrode of each TFT is connected to the sense electrode of the sense element. The sense electrodes together with overlying dielectric material and individual fingerprint portions when placed over the dielectric material, constitute capacitors. The row address conductors are connected to a scan circuit which applies a gating (selection) signal to each row conductor in a respective row address period to turn on the TFTs of the sense elements of each row in sequence. Simultaneous with a gating signal, a predetermined potential is applied to the column address conductors to charge the capacitors. The individual capacitances of these capacitors depend on the spacing of the fingerprint portions from the sense electrodes, as determined by the presence of a ridge or a trough of the fingerprint, and are measured by sensing the charging current flowing in the column conductors during charging of the capacitors, using current or charge sensing amplifier circuits incorporated in the drive circuit. At the end of the row address period, the TFTs are turned off and a gating signal applied to the next row conductor to turn on the TFTs of the next row of sense elements. Each row of sense elements is addressed in this manner in turn and the variation in sensed capacitances produced over the array of sense elements by a fingerprint ridge pattern provides an electronic image or representation of the three dimensional form of the fingerprint surface.
In order to allow consecutive readings of the capacitance image of a fingerprint, or readings of different fingerprints, in successive field scan operations, the charge on the sense electrodes is removed, or at least reduced, before the sense elements are addressed again. This is achieved either by incorporating a resistor in each sense element which is connected between the sense electrode and ground, by changing the predetermined voltage applied to column conductors in successive read cycles, or by arranging the drive circuit to include an intermediate reset cycle between successive read cycles. The provision of a resistor, for example using a doped semiconductor material, is difficult and complicates the fabrication of the sense element array, while the other two discharge schemes cause complications to the drive circuit. Moreover, it is important in this device for minimizing cross-talk problems that there is highly controlled, or low, leakage in the TFTs and that the leakage characteristics of the TFTs across the array are substantially uniform. This can be difficult to achieve, particularly if the TFTs are formed on a polymer substrate. The speed at which successive read-outs could be achieved would also be compromised by the need for these discharge schemes.